A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”—direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is well-known in the art of projection lithography that during exposures, elements in the projections system absorb radiation, heat-up and therefore introduce aberrations into the projections system, resulting in reduced image quality at substrate level. These effects are particularly acute when using illumination modes, such as dipole illumination and quadrupole illumination, in which the intensity distribution in the pupil plane of the illumination system and/or deep ultraviolet radiation (DUV), e.g. at 198, 157 or 126 nm, since the choice of materials from which lenses useable with these wavelengths can be made is quite limited and even the best materials have significant coefficients of absorption at those wavelengths. The problem particularly affects projection systems formed by refractive lens elements and is therefore often referred to as lens heating. Even with cooling systems to maintain the projection system at a constant temperature, sufficient local temperature variations can occur to cause noticeable loss of imaging quality.
Therefore, many projection systems in lithographic projection apparatus are provided with one or more actuated adjustable elements whose shape, position and/or orientation in one or more degrees of freedom can be adjusted during or between exposures to compensate for lens heating effects. A computer model predicts the lens heating effects that are expected and calculates appropriate corrections to be effected by the adjustable elements. Prior art computer models have calculated the lens heating effects in terms of Zernike polynomials describing the aberrations in the pupil plane of the projection system and applied corrections via control “knobs” on the projection system that adjust one or more adjustable elements to give a correction corresponding to the relevant Zernike polynomial. However, prior art lens heating correction methods have not always been completely effective and some residual aberrations often occur.
Other attempts to deal with the problem of non-uniform lens heating include the provision of additional light sources, e.g. infra-red, to heat the “cold” parts, i.e. those not traversed by the intense parts of the projection beam, of elements of the projection system, see U.S. Pat. No. 6,504,597 and JP-A-08-221261. The former reference addresses non-uniform heating caused by a slit-shaped illumination field and the latter references addresses non-uniform heating caused by zonal or modified illumination. The provision of such additional light sources and guides to conduct the additional heating radiation to the correct places may increase the complexity of the apparatus and the increased heat load in the projection system necessitates the provision of a cooling system of higher capacity.